Electron beam source assemblies, particularly electron gun assemblies used in the field of thin film deposition, are known in the prior art. Such assemblies typically utilize a thermionic emitter mechanism to generate and accelerate an electron beam. The accelerated electron beam is deflected by a magnetic field into a crucible structure containing one or more materials. The material(s) are vaporized by the electron beam and deposited on the desired substrate. One such electron beam source assembly is disclosed in U.S. Pat. No. 3,883,679 to Shrader et al.
The electron beam source assemblies described above, including the Shrader device, typically form the water cooled crucible in a metallic block, place the emitter mechanism at a distance from the crucible to prevent damage to the emitter, and position permanent or electromagnets to generate a transverse magnetic field to deflect the electron beam and focus it onto the crucible.
The transverse magnetic field used to deflect the electron beam is typically generated between and perpendicular to parallel pole pieces extending from a single magnet, or between spaced, parallel magnets. The transverse magnetic field generally includes uniform field lines between the magnetic pieces, plano-convex field lines above and at the ends of the magnetic pieces, and convex field lines above the plano-convex field lines.
The magnetic structure employed in electron gun assemblies as described herein typically generates field lines at the emission site of the electron gun assembly to contain the emitted beam within the assembly and prevent loss or dissipation of the beam therefrom. The beam is typically injected from the emitter into a recess formed in the metallic block and subject to the magnetic field where it generally passes first, through uniform field lines, then through plano-convex and convex field lines, and again through uniform field lines surrounding the crucible. The passage of the beam through such changing magnetic field lines causes the electron beam to diverge and re-converge several times along its trajectory thereby creating several focal points. The establishment of multiple focal points makes the beam difficult to focus and to control. When it is desired to move the beam, the changing field lines and multiple focal points make it difficult to predict the size, shape and density of the newly positioned beam.
The difficulty described above in controlling and focusing electron beams is compounded by the typical architecture of the prior art emitter mechanism. As shown in the Shrader patent discussed above, the emitter mechanism of electron guns used for vaporizing materials in a high vacuum environment typically includes an electron emitting filament partially shielded by a beam former and by the anode structure. The negative potential at the shielding beam former and the anode generate resistance to electron emission from the filament. When electrons are emitted from the partially obstructed filament into a high negative potential, they are initially directed downward into an open cavity of high negative potential containing the cathode structure which typically defines an irregular surface with notches, bolts and shelf areas. Thus, the electron beam in prior art emitter assemblies is emitted downwardly through irregular electrostatic field lines before it rises towards the anode and encounters the variously shaped magnetic field lines discussed above. The architecture of prior art electron gun assemblies, together with the differing magnetic field lines of the magnetic structures used to deflect the electron beam, make it difficult to control and focus the beam upon the source material in the crucible.
Attempts to solve the problems of focusing and control of electron guns used for vaporization in high vacuum environments have centered primarily upon efforts to continue re-converging the electron beam by varying the magnetic structures in the assemblies. Several examples include two Hanks patents, U.S. Pat. Nos. 4,835,789 and 4,947,404. The Hanks devices use a plurality of small, horizontal or vertically aligned magnets placed around the crucible. The magnets may be moved as needed to control the electron beam.
Other attempts to control the beam include lowering the position of the crucible within the electron gun assembly to confine the generally circular or oval trajectory of the beam. When the crucible is below the top surface of the electron gun assembly, vaporized material solidified on the cooler lip area formed above the crucible may flake off and fall into the crucible thereby adversely affecting thin film deposition. Additionally, lowering the crucible into the assembly subjects the crucible and electron beam to further undesirable repelling magnetic field lines from the magnetic structure. Repelling magnetic field lines around the crucible cause the electron beam trajectory to circle toward the crucible walls thereby undesirably increasing the temperature on the walls. The Shrader patent discussed above added magnetic shielding in an attempt to prevent unwanted field lines from affecting the crucible. Additional magnetic structure further complicates the magnetic field lines of the assembly thereby making it more difficult to predict the effects of moving the beam around the crucible.
As can be seen from the discussion of the prior art, an unsolved need exists for an improved electron gun assembly for vaporization of substances in high or ultra high vacuum environments, the assembly increasing electron emission and providing a magnetic structure designed to create a focusable, moveable electron beam with an improved trajectory travelling to and passing through the crucible, and providing an uniform image of the beam emitter at the target.